1. The Technical Field
The present invention relates to solid oxide fuel cells (SOFCs), and the structures and materials by which they are constructed. In particular, the present invention concerns anode structures for solid oxide fuel cells.
2. Solid oxide fuel cells are recognized as having the potential to mitigate environmental problems while meeting the power generation and cogeneration needs of tomorrow. Present work in SOFCs is centered on the use of zirconia electrolytes operating at high temperatures between 900-1000.degree. C. These high temperatures present special challenges related to materials degradation.
The high temperature is particularly challenging to anode stability. Metallic nickel phase appears to be by far the best anode material for fuel oxidation. It is also preferred for hydrocarbon fuel since nickel is a good catalyst for hydrocarbon reformation. The primary drawback in using a nickel anode is that a nickel anode will typically have a high thermal expansion coefficient, on the order of 13.times.10.sup.-6 cm/cm/C, compared to the commonly used electrolyte material, zirconia, which has a thermal expansion coefficient of 10 to 11.times.10.sup.-6 cm/cm/C. Furthermore, metallic nickel exhibits poor wetting characteristics on zirconia. Both these issues promote coarsening of nickel particles leading to eventual loss of both physical contiguity (which lowers electrical conductivity) and reaction interface area (which slows electrode kinetics).
To mitigate some of these issues a mixture of nickel and zirconia (30 to 50 percent zirconia by volume has been used as the anode to provide a closer thermal expansion between the anode and the electrolyte, as well to increase the three phase boundary (where the gas phase-ionic conductor [zirconia] and electronic conductor [nickel] meet). Although the bulk thermal expansion of the anode and electrolyte, respectively, may be closer, at the microstructural level, the thermal expansion mismatch between individual nickel and zirconia grains does still exist.
Additionally, the typical fabrication processes used to apply the anode to the electrolyte, such as screen printing, plasma spraying or slurry coating of the anode (mixture of nickel oxide and zirconia) onto a densified zirconia substrate and sintering the anode are unlikely to provide a contiguous zirconia structure to provide an enhancement in the three phase boundary area. A high sintering temperature, 1400 to 1500.degree. C., helps in providing the necessary sintering of zirconia particles in the anode mixture to accomplish a contiguous structure. However, the thermal expansion mismatch typically warps the substrate at those sintering temperatures. In addition, the nickel particles in the nickel-zirconia mixture still coarsen over time resulting in cell performance degradation.
The ideal anode microstructure has sufficient open porosity for reactant access from the bulk fuel gas to the electrolyte-anode interface and removal of product species from the interface; and a mechanically stable microstructure to maintain continuity, preferably of both electronic and ionic conducting phases and a well adhered microstructural interface. It is thus necessary to reduce the coarsening rate of the nickel particles to improve stability of cells. It is also necessary to promote the wettability of (or adhesion of) the Ni on to the zirconia substrate.